This is a multi-group research study which plans to evaluate the overall progression-free survival of children with average-risk medulloblastoma treated with craniospinal radiation and local boost radiotherapy plus one of two adjuvant chemotherapy regimens differing in the replacement of CCNU by cyclophosphamide. The long-term neurocognitive, endocrinologic and cardiopulmonary sequelae of radiotherapy with adjuvant chemotherapy will be determined, as well as the feasibility of routine surveillance scans to detect sub-clinical recurrent disease. Another objective of this study will be to evaluate the sensitivity of molecular and biochemical techniques to predict progression-free survival and disease relapse.